1. Field of the Invention
The present invention relates to a method of deciding a measurement value, and, more particularly, for deciding the quality of the measurement value obtained by measuring the line width, the line interval or the like of a pattern on an object to-be-measured.
2. Description of the Related Art
Heretofore, an automatic measurement, which is performed with a SEM (scanning electron microscope) for the line width, the line interval or the like of a microscopic pattern that is formed on a mask for transferring the microscopic pattern onto a wafer, has sometimes turned out a failure from various causes. The “failure” termed here signifies that, although a numerical value being the result of the measurement is obtained, the value is incorrect by any cause. In the automatic measurement in the absence of an operator, it is often the case that the cause cannot be easily grasped. This situation occurs for reasons as stated below.
(1) Since the measurement value itself exhibits a variance, a correct value is not fixed.
(2) The measurement value sometimes deviates widely due to the failure of autofocusing in the SEM by any cause, or due to the deviation of an image attributed to charging-up. In some cases, the correct value is obtained by trying the autofocusing again.
(3) A pattern dimension sometimes deviates more than is expected. This dimension is correct as the measurement, and ought to be marked as an abnormal place.
It is required of an automatic measurement apparatus to automatically discriminate these reasons and perform appropriate processing.
An object to be measured here is-a length (width) which is measured along a certain direction (for example, X-direction or Y-direction) in a certain place of the pattern. The pattern is based on a line which has a finite width (including also the interval between lines, and termed “line” or “space”), and a rectangle which occupies a finite area (usually termed “contact hole” or “dot” in the pertinent technical field). In a case where the pattern has a complicated shape, the length (width) to be measured becomes the width of the pattern along a specified line. In case of the contact hole, it is also included to measure the area.
A signal intensity distribution along a designated direction is extracted from the SEM image of the pattern to-be-measured, and both the end edges of the pattern are detected from the signal intensity distribution, so as to calculate the width.
There are several methods for determining the positions of the edges on this occasion.
(1) Threshold value method: In a case where the intensity distribution in the vicinity of each edge is normalized and where the width between the minimum and the maximum is set at 1 (one), the point of an intensity having a threshold value x (1.0≧x≧0) is set as the edge position.
(2) Linear regression method: Regression lines are respectively obtained in a partial intensity distribution near the edge and in a partial intensity distribution near the minimum point, and the intersection point between the regression lines is set as the edge position.
(3) Peak method: A line width is calculated with tentative edges being the peak positions of intensity distributions in the vicinity of the edges, and it is corrected by calibration.
(4) Other (combination): An edge intensity distribution is subjected to a function approximation, and the edge position is determined in combination with the threshold method.
Whether or not the dimension measured by any of the above methods (1) through (3) is correct, has hitherto been decided by either of methods stated below.
(a) A person observes the image of the object to-be-measured, and decides whether or not the measurement proceeded correctly. (Automation is impossible.)
(b) A certain range of measurement values as centers round an expected dimension is set, and any measurement value outside the range is regarded as the failure of the measurement. (Automation is possible.)
Here, the method (a) has had the problems that the automation is impossible, and that individual differences ascribable to the deciders cannot be excluded.
On the other hand, in case of the method (b), the automation is possible, and the predetermined criterion is applied, so that objective data are obtained. This method, however, has had the problem that the simple setting of the range cannot cope with the variance of the object to-be-measured, a systematic change such as process bias, or the like.